The journals of the Lewis and Clark expedition contain the oldest meteorological records for Oregon. When the explorers camped here in the winter of 1806, their diaries contained numerous brief entries that are nearly identical: "hard rain all last night, and continues as usual." Or, "It rained hard all last night, & still continued the same this morning." And, "It continued Raining during the whole of this day."

Most people still see Oregon as a damp, rainy place. But Oregon isn't a sponge that can absorb winter rains and then release the supply gradually through the summer just when we happen to need more water. Our summer supplies are dependent on the gradual melting of mountain snows and the storage capacity of artificial reservoirs. Together, these factors have created an illusion of plenty, suggesting that easy water is always at hand.

In fact, supply falls short of demand virtually every year. Between 1990 and 2000, many critically important Oregon rivers had so much water taken out that their flows fell below the legal minimum. This isn't a rare event. In 2000, one-third of our critical rivers failed to have sufficient flow during every month of the year.

So why can't you look out your window and see an empty river? For one thing, most of our streams and rivers are located in rural areas, and the vast majority of Oregonians (over 70%) live in urban areas. The separation of wet and dry, urban and rural, is also made more distinct by the Cascade Mountains. These mountains create a barrier that makes water-bearing clouds drop most of their moisture in the heavily populated western half of the state. As a result, the western side gets 4 to 7 times more annual precipitation than the sparsely populated eastern half of the state.

Taken as a whole, the state averages 28 inches of precipitation each year (about the same as Texas). The average annual runoff is 20 inches. That leaves us with just 8 inches a year - on average. It would take just a few consecutive dry years to put Oregon into a water emergency.

Water that arrives as rain or snow becomes part of our surface water supply. Some surface water is captured in reservoirs for future use. A lot of it is used for irrigation, hydropower and public water supplies. But eventually, most of it flows to the sea.

Our other water supply is located in underground aquifers. This water has taken hundreds of years to filter down from the surface, through layers of rock, until it was trapped in porous layers of earth. Like most states, Oregon uses groundwater to supplement surface water supplies. But we are using it faster than nature can replace it. The aggressive use of groundwater in recent years has caused the water table to drop drastically. Now people have to dig very deep wells just to reach water that once was easily attainable.

Part of Oregon's water problem is due to a basic lack of facts. We don't know how much water is in the ground. And we don't know how much water is actually being used — whether groundwater or surface water. But we do know that demand exceeds annual supply.

To put this in familiar terms, imagine that Oregon's water supply is a checking account. To start with, we don't know how much money we have. We are writing lots of checks without balancing the account. We're also borrowing lots of money without considering the consequences. Sooner or later, our cash and credit will be exhausted. What then? Clearly, Oregon needs to be more careful about how we spend our water.

Although water still comes out of your faucet, our "invisible" water shortage has already produced a variety of secondary problems that often go overlooked. For example, when there isn't enough water to dilute pollutants and waste, concentrations can reach unhealthy levels. In 2000, a quarter of all Oregonians used public drinking water that failed to meet health standards.

Fish and wildlife are also affected by shortages. If streambeds are pumped dry, fish can't reach spawning grounds. If there isn't enough water to provide deep, cool pools, stream temperatures can get too warm for fish to survive. These are just two of the water-related factors that have contributed to the loss of Oregon's native fish stocks.

There are dozens of similar symptoms that prove Oregon has a serious problem. But how did we get to this point? The underlying cause is fairly basic: too many agencies have promised too much water ? to too many people ? for too many purposes. Now there isn't enough to go around — and no one is willing to give up their legal entitlements.